1. Technical Field
The present invention relates to a method of efficiently recovering gold from an aqueous solution such as a post-extraction liquid containing a low concentration of gold, based on the solvent extraction process.
2. Related Art
In copper smelting, recovery of a trace amount of gold contained in the concentrate is an inevitable issue. The concentrate is processed by dry smelting or wet smelting, wherein various methods for recovering gold have been investigated for both types of smelting.
In the dry smelting, the concentrate is generally melted at high temperatures typically at 1200° C. or above, so as to produce a crude copper having a purity of approximately 98%, followed by electrolytic refining to thereby produce an electrolytic copper having a purity of 99.99%. Gold in the concentrate migrates into the crude copper, and is then concentrated in a precipitate called anode slime, which produces in the process of electrolysis. Recovery of gold starts from the anode slime, and is proceeded typically by combination of chloride leaching and solvent extraction.
On the other hand, in the wet smelting, copper in the concentrate is generally leached in a solution in a sulfuric acid bath or a chloride bath, typically followed by a combined process of solvent extraction and electrowinning, to thereby produce an electrolytic copper. Gold in this process does not concentrate or precipitate unlike in the dry smelting, and instead remains in a leaching residue and in a leachate while being not concentrated.
Typically as described in Japanese Laid-Open Patent Publication No. 2010-180450, gold remained in the leaching residue is sieved into an on-screen fraction and an under-screen fraction, and the under-screen fraction is then subjected to flotation, to thereby recover gold into the float, and processed by an additional dry or wet process.
Gold which resides in the leachate is recovered generally by adsorption on activated carbon, for the reason described below.
Techniques adoptable to separation of metals according to the wet process include ion exchange, activated carbon adsorption, electrowinning, cementation, and solvent extraction. Each process has an concentration range adapted thereto, wherein the activate carbon adsorption is generally adoptable to a solution having a metal concentration of several grams per liter or lower, and the solvent extraction process is adoptable to a solution having a metal concentration of several grams per liter or higher.
Gold concentration in the leachate which contains gold leached from ores is generally low. The gold concentration of the leachate obtained from the above-described copper smelting based on the wet process is 10 mg/L (0.01 g/L) or lower. Accordingly, the activated carbon adsorption process is generally adopted to separate gold from the copper leachate.
Gold adsorbed on activated carbon is eluted by a solution of cyan compounds, thiourea or thiosulfuric acid which readily form a complex with gold, and then recovered by cementation typically using zinc, or electrowinning. Alternatively, the activated carbon having gold adsorbed thereon is directly combusted.
Cyan compounds, thiourea and thiosulfuric acid are, however, expensive. In particular, cyan compounds are known to be toxic, so that safety and environmental impact of waste liquid treatment after the elution process remain to be solved. Combustion of the activated carbon costs high since the activated carbon is not recyclable, and additionally needs waste gas treatment since the adsorbed metals other than gold are combusted together.